The reflectarray is an alternative to directly-radiating phased array antennas and promises higher efficiency at reduced cost. A key advantage of reflectarray antennas over conventional phased arrays is elimination of the complex beam-forming manifold and costly transmit/receive modules. The reflectarray is also reciprocal—the same aperture can be used for transmit and receive functions. In 1963, Berry, Malech and Kennedy introduced this new class of antennas that utilized an array of elementary antennas as a reflecting surface.
In 1976, Phelan patented (U.S. Pat. No. 3,949,407) a scanning reflectarray based on interleaved Archimedian spiral antennas. Spiral arms were interconnected with diode switches. The spirals are inherently circularly polarized over a broad bandwidth. (i.e., the far-field phase shift from a circularly polarized radiator is proportional to the apparent physical rotation of the radiator.)
In 1978 Malagisi proposed a microstrip reflectarray. In a microstrip reflectarray, stubs aligned with the desired polarization direction and of varying length are attached to the elements to effect phase shift. Incident energy from the primary feed propagates down the stub, where it reflects from the open (or short) end, and re-radiates with a delay corresponding to twice the electrical length of the stub.
A circularly polarized microstrip reflectarray with a 55% efficiency was reported by Huang and Pogorzelsk in an article entitled Ka-Band Microstrip Reflectarray with Elements Having Variable Rotation Angles, IEEE Transactions On Antennas and Propagation, Vol. 46, No. 5, May 1998. The antenna used square patches with identical stubs but varying rotation angles. Huang discloses a means of achieving cophasal far-field radiation for a circularly polarized microstrip reflectarray with elements having variable rotation angles. Two Ka-band half-meter microstrip reflectarrays were fabricated and tested. One of the arrays was of conventional design having identical patches with variable length microstrip phase delay lines attached. The other array had identical square patches with identical microstrip delay lines but different element rotation angles. The element with variable rotation angles resulted in better performance according to Huang.
In 2000, Romanofsky and Miranda disclosed a scanning reflectarray antenna based on thin-film ferroelectric phase shifters. None of these technologies provided a practical or cost effective means to replace parabolic reflectors intended for communications with geostationary satellites. The current state-of-practice is to use a solid parabolic reflector which must be physically pointed directly at the satellite in order to establish a communications link.
U.S. Pat. No. 6,081,235 to Romanofsky et al. disclosed a corrugated feed horn attached to nonmetallic struts situated at the virtual focus of the antenna. Further, the '235 patent to Romanofsky et al. states that “[t]he incident circularly polarized signal is absorbed by each element of the reflectarray, routed through the stubs, which are in turn connected to the phase shifters, and re-radiated with a phase shift equal to twice the electrical length of the stubs-coupled electrical lines arrangement. By varying the bias across the coupled lines of each element, the appropriate phase shift can be attained, for electronic scanning without any physical movement of the antenna to produce the desired beam steering.” The row-column steering concept is a way to cut manufacturing cost but limits field of view. In the '235 patent the cellular array compensates for the spherical phase from the feed by tuning ferroelectric phase shifters.
U.S. Pat. No. 6,384,787 to Kim et al. discloses a flat reflectarray antenna utilizing a polarization twist function and predetermined phase shifts to provide a directed narrow beamwidth signal as set forth in col. 1 lns. 5-8. It is apparent that Kim et al. does not apply to circular polarization, cellular implementation, or thick, high dielectric constant substrates.
Several concepts for reflectarrays have been proposed but the usual context has been as a replacement for a parabolic dish that is mechanically pointed to a target or a as a competitor to directly radiating Gallium Arsendie Monolithic Microwave integrated Circuit phased arrays.
FIG. 10 is a prior art schematic 1000 of a patch antenna 102 fed orthogonally with microstrip lines for the purpose of evaluating the polarization of the reflected field. In principle, the image can be cross-polarized with respect to the desired beam. Consider the simplified schematic of a patch antenna attached to orthogonal microstrip lines feeding some type of combiner that ostensibly leads to a variable phase shifter as shown in FIG. 10, where Δx=Δy+π/2. In practice, a quadrature (90°) hybrid coupler or equivalent would be used to couple the patch to the phase shifter. The reflectarray is in the X-Y plane. Assume that the incident wave is in the minus z direction and right hand circularly polarized (RHCP) such that Einc=(jux−uy)ei(βz-ωt) where ux and uy are unit vectors in the x- and y-directions respectively. Ignoring the time dependency, the reflected field is Ere1=(−jux−uy)ej(2βΔy-βz) and the electric field vector angle is easily shown to be proportional to ωt so it is likewise RHCP. Phase shifter contributions are neglected. The signal reflected from the ground plane will be LHCP due to the reversal of propagation direction. It can be shown that, in general, if one arm of the patch is 90 degrees longer than the other, the reflected signal will have the same sense polarization as the incident signal.